Williams syndrome (WS) is a genetically based disorder associated with a distinctive social and behavioral phenotype, characterized by a mixture of unique aspects of sociability (e.g., keen social attention, overfriendliness toward strangers, empathy), and impairments commonly found in other populations with mental retardation (e.g., in theory of mind, social perception and cognition, in socio-communicative behavior). Although recent studies have focused on defining the social-perceptual and social-cognitive abilities of people with WS, a comprehensive characterization of their social-behavioral profile and its developmental trajectory has not yet been achieved. The goal of the proposed research is to investigate the developmental roots of the apparently contrasting aspects of the social-behavioral phenotype of WS. In this pilot investigation we plan to focus on how three essential components involved in early socio-affective functioning - attachment relations, temperament, and style of interactive behavior - relate to distinct patterns of social behavior in WS, specifically, to empathic responsiveness. We hypothesize that the roots of the unique empathy documented in people with WS lie in their atypical approach-behavior toward strangers, coupled with a disinhibited personality. Children with WS aged between 30 and 42 months will be compared to age- and developmental level-matched children with Down syndrome and to typically developing children matched on age, in a series of assessments of attachment relations, temperament, style of dyadic interaction, and empathic responsiveness in different contexts. These assessments will involve a combination of laboratory observations based on semi-structured activities with familiar and unfamiliar social partners (adults and peers), and questionnaires and interviews with children's primary caregivers. To capture developmental relations between these domains, assessments of attachment, temperament, socio-interactive behavior and cognitive-linguistic abilities in Year 1 will be followed one year later with assessments of socioemotional competence and empathy. We predict that children with WS will show distinctive behavioral patterns in these domains of social functioning, but that some of these differences will be mediated by the social context in which the behaviors occur and the time of assessment. The results of this investigation will provide preliminary data for a model of the developmental trajectory of social-affective functioning in WS, which might begin to explain the unique patterns of social behavior and social understanding in this distinctive syndrome, and may have implications for designing intervention strategies to enhance social skills in children with developmental disorders.